What kind of problems are you having and are you having most of your problems over wireless? There are a variety of things that could be happening here:
(1) someone could be using your wireless connection or something could be interfering with it. 2.4ghz wireless is interfered with by bluetooth, including your controllers, some cordless phones/baby monitors, other wireless access points (only 3 channels are orthogonal -- the rest interfere), microwaves, florescent bulbs, etc.
Always use wired where possible, of course. It's better in every way except ease of physical connections.
(2) Xbox Live requires you to forward ports to your Xbox 360 to have an "open" connection. Your router probably only allows you to forward these ports for one of your Xbox 360s. Make sure you are getting an open connection on whichever 360 you are using for Live. You can check this in the dashboard under system->network.
(3) Someone on your network could be uploading a lot which would choke your connection (the network needs to send acknowledgments that it gets information -- if those are delayed, everything will suck). Ensure that none of the machines are running p2p applications like bittorrent or kazaa.
One way to start is to disconnect everything but your Xbox 360 on Live! and test that. If it's good, then the problem could be coming from one of the other machines.







